Chou C L, Knepper M A
Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
Am J Physiol. 1993 Feb;264(2 Pt 2):F337-43. doi: 10.1152/ajprenal.1993.264.2.F337.
We measured the urea and NaCl permeabilities (Purea and PNaCl, respectively) of the following nephron segments from chinchilla: the upper part of the long-loop descending limb (from outer medulla, LDLu), the middle part of the long-loop descending limb (from outer 30% of the inner medulla, LDLm), the lower part of the long-loop descending limb (from deep inner medulla, LDLl), and the thin ascending limb (from deep inner medulla, ATL). We found that Purea (x10(-5) cm/s) was relatively low in the LDLu (3.3), but that the value was larger in the inner medullary thin descending limb (16.8 for LDLm and 47.6 for LDLl). The ATL had an even higher value (170). Phloretin, 0.25 mM, added to the peritubular bath had no effect on Purea of these segments, suggesting that the rapid transport rate is not due to a phloretin-sensitive facilitated transport pathway like that seen in the inner medullary collecting duct. PNaCl (x10(-5) cm/s) also increased with distance along the length of the thin descending limb (LDLu, 11.7; LDLm, 41.2; LDLl, 98.4; and ATL, 321). Calculations from NaCl dilution potential measurements showed that LDLu was Na+ permselective, whereas LDLl and ATL were Cl- permselective. High solute permeabilities in the inner medullary thin descending limb contradict a major requirement of the passive model of urinary concentration developed previously (J. P. Kokko and F. C. Rector, Jr. Kidney Int. 2: 214-223, 1972; and J. L. Stephenson. Kidney Int. 2: 85-94, 1972).
我们测量了来自南美栗鼠的以下肾单位节段的尿素和氯化钠通透性（分别为(P_{urea})和(P_{NaCl})）：长袢降支上段（来自外髓，LDLu）、长袢降支中段（来自内髓外30%，LDLm）、长袢降支下段（来自内髓深部，LDLl）以及细升支（来自内髓深部，ATL）。我们发现，(P_{urea})（(×10^{-5}) cm/s）在LDLu中相对较低（3.3），但在内髓细降支中该值较大（LDLm为16.8，LDLl为47.6）。ATL的值更高（170）。添加到肾小管周围浴液中的0.25 mM根皮素对这些节段的(P_{urea})没有影响，这表明快速转运速率并非归因于像在内髓集合管中看到的那种对根皮素敏感的易化转运途径。(P_{NaCl})（(×10^{-5}) cm/s）也随着细降支长度的增加而升高（LDLu为11.7；LDLm为41.2；LDLl为98.4；ATL为321）。根据氯化钠稀释电位测量结果计算表明，LDLu对(Na^{+})具有选择性通透，而LDLl和ATL对(Cl^{-})具有选择性通透。内髓细降支中高溶质通透性与先前提出的尿液浓缩被动模型的一个主要要求相矛盾（J.P. Kokko和F.C. Rector, Jr. 《肾脏国际》2: 214 - 223, 1972；以及J.L. Stephenson. 《肾脏国际》2: 85 - 94, 1972）。
